The use of coordinate measuring machines (CMM) for measuring coordinates of a workpiece surface of at least one workpiece is known. The workpiece surface is scanned with at least one sensor of the CMM, specifically by way of tactile probing of the workpiece surface using a probe, and/or in a contactless manner. Contactless scanning sensors include optical sensors. Laser triangulation sensors are a type of optical sensors. Confocal white light sensors are another type of optical sensors. Their use as sensors of coordinate measuring machines is known, e.g., from DE 103 40 803 A1.
A coordinate measuring machine is a machine that can measure coordinates of a workpiece using at least one sensor. The present invention specifically relates to coordinate measuring machines that can measure coordinates of surfaces and/or material boundary surfaces of workpieces. The present invention furthermore specifically relates to coordinate measuring machines having a movement apparatus that permits a relative movement of the sensor and of the workpiece. One possibility for moving the sensor and the workpiece relative to one another is offered by CMMs having one or more sensors that are movable relative to a base at rest. Examples are coordinate measuring machines having a portal design or a gantry design. The workpiece to be measured is typically placed directly onto the base at rest, e.g., a measurement table, or by a workpiece holder (e.g. a rotary table) on the base. Another possibility for a relative movement of sensor and workpiece is offered by coordinate measuring machines having a movable measurement table and a fixed sensor, e.g., CMMs having a XY measurement table, which is movable in two movement directions that are perpendicular to one another. As is shown by the example of the CMM with a portal design or gantry design having a rotary table, mixed forms of both types of CMM are possible, i.e., the relative movement can be brought about both by a movement of the sensor and by a movement of the workpiece relative to a base at rest. A further such mixed form is realized, e.g., by the O-INSPECT production series from Carl-Zeiss Industrielle Messtechnik GmbH, Oberkochen, Germany. The workpiece is placed on a movable XY measurement table. However, the sensor is additionally movable in the vertical direction (Z-direction). The invention relates in particular to all these types of CMM.
A confocal white light sensor is a sensor that uses the principle of chromatic confocal distance measurement. White light (i.e., electromagnetic radiation, not necessarily visible, having radiation components of a plurality of wavelengths) is radiated by a light source onto a focusing optical unit. The focusing optical unit effects dispersion of the radiation, i.e., chromatic aberration occurs. As a result, the radiation components of the different wavelengths are focused at different distances to the focusing optical unit. If an object that reflects radiation back in the direction of the sensor is located in the respective focus (focal point or focal line), the sensor detects radiation of the wavelength with maximum intensity that was back-reflected in the focus. It is also possible that radiation of different wavelengths is back-reflected at the same time at their respective foci. In that case, the sensor detects in each case a (local) intensity maximum at these wavelengths. If the distance from the focus to the sensor (e.g., to the focusing optical unit) is known, it is possible to ascertain the distance between the sensor and the workpiece surface from the wavelength of the single intensity maximum or from the wavelengths of the intensity maxima. However, this knowledge does not initially exist and is generally obtained by way of a reference measurement, in which the distance between the sensor and the reflection location is also measured in a different manner, e.g., by using a laser interferometer.
United States Patent Application Publication No. 2016/0076867 describes the automatic receiving of white light sensors of a coordinate measuring machine. A movable part of the coordinate measuring machine and the white light sensor each have an interchange interface for coupling the white light sensor to a carrier structure of the coordinate measuring machine. Consequently, various sensors can be operated successively at the interchange interface of the CMM.
White light sensors are typically combined with a signal processing unit that processes the signals of the white light sensor. In particular, evaluation of the signals of the white light sensor takes place in the signal processing unit, which means that the unit is an example of an evaluation device. If the above-mentioned knowledge about the distance of the focus in each case for the individual wavelength components of the white light is available to the evaluation device, the evaluation device can also ascertain the actual distance of reflective surfaces and material boundary surfaces at the transition between different materials from the sensor signals. The white light sensor and the evaluation device are typically connected together using a fiber-optic cable, i.e., the signal supplied by the white light sensor is the radiation that is received by the sensor, in particular reflected by a workpiece. This has the advantage that heat produced during operation of the evaluation device is not produced at the location of the white light sensor. The white light can also be transferred from the light source to the location of the focusing optical unit of the white light sensor via a fiber-optic cable. In more general terms, a light guide is used for the radiation received by the white light sensor and/or for the white light from the light source. As already mentioned, the white light does not have to be, or does not have to be completely, in the visible wavelength range. When using a light guide, at least parts of the sensor system that measures the distance between the sensor and the workpiece are therefore not situated at the location of the focusing optical unit, which is also the location where the reflected radiation is received. In particular when using a white light sensor on a CMM having a movement device that moves the sensor relative to a base at rest, it is possible for the movable white light sensor to merely have the focusing optical unit that also receives the reflected radiation and has, e.g., an interface with or a fixed connection to a light guide. Parts of the light guide and the evaluation device (and the light source, to the extent that it is not part of the movable sensor) can be stationary with respect to the base. The sensor can therefore also be referred to as a sensor head of the sensor system.
As mentioned, the knowledge of the distance of the focus to the sensor must be obtained for every wavelength or frequency of the measurement radiation of the sensor, i.e., for every component of the white light that is to be used for the distance measurement. It would be possible in theory for this knowledge to be obtained by precisely taking account of the optical properties of the focusing optical unit of the sensor. The focus distances could be calculated therefrom. However, this is not practical due to manufacturing tolerances. The optical properties of the focusing optical unit can also depend on the operating conditions, in particular on the temperature. In practice, the knowledge of the wavelength-dependent focus distance is therefore already obtained at the site of the manufacturer of the sensor by way of a reference measurement: For a large number of distances between the sensor and a reference body, which reflects the measurement radiation back onto the sensor, first the measurement signals of the sensor are obtained and recorded and/or evaluated, and, in addition, the distance between sensor and reference body is measured in an additional distance measurement of a different distance measurement system. This makes it possible in particular for linearization parameters to be determined, with which linearization parameters it is possible to ascertain for an object distance of varying size function values of a linear mathematical function of the measurement value (i.e., of the measurement result of the sensor) depending on the object distance based on measurement signals of the confocal white light sensor. In simplified terms, the linearization parameters allow the measurement signals of the sensor (which reproduce in particular the intensity distribution of the reflected light received by the sensor over the wavelength range of the measurement radiation) to be converted into distance measurement values of the object distance such that, for twice the object distance, twice the measurement value of the object distance is also obtained. These measurement values of the object distance obtained can then be output by the evaluation device. During the measurement operation of the sensor, the user, or a measurement processing device that is connected to the evaluation device, therefore obtains merely the measurement values obtained by way of the knowledge of the focus distance. The primary measurement signals of the sensor are typically not output from the sensor system during the normal measurement operation.
Confocal white light sensors are highly resolving, and accurate distance sensors as compared to other distance sensors (such as capacitive sensors). By way of example, white light sensors having a maximum resolution of a hundredth of a micrometer and measurement regions in the order of magnitude of several tenths of millimeters to several tens of millimeters are available on the market. One example of this is the confocal white light sensor with the type designation “Confocal IDT IFS 2405” from Micro-Epsilon Messtechnik GmbH and Co. KG, Ortenburg, Germany.
If white light sensors, as described in United States Patent Application Publication No. 2016/0076867, can be interchanged on a coordinate measuring machine via an interchange interface such that they are available for use on the CMM, their operation necessitates the exact knowledge of the focus distance. However, as has likewise already been mentioned, it is advantageous to locally decouple the evaluation device and/or the light source from the white light sensor (i.e., from the measuring head). Only the white light sensor is arranged on the CMM by the interchange interface. Interchanging of a white light sensor therefore results in an operational state only if the evaluation device and/or light source is also present at a suitable location of the CMM or in the vicinity thereof and is connected to the sensor by a light guidance. However, the knowledge of the focus distance or the corresponding parameters that have been provided by the manufacturer of the sensor are in any case valid as regards the measurement accuracy and reproducibility of the measurement stated by the manufacturer only if the sensor system is in the same overall state as it was during the reference measurement at the site of the sensor manufacturer. Even if the evaluation device and/or the light source is not changed and, e.g., remains mounted on the CMM for a prolonged period of time, while the sensor is decoupled from the CMM and then again coupled to it, this can change the operating state of the white light sensor system. In particular, the light guide can be attached at the interface to the sensor and/or to the evaluation device and/or light source in a position and orientation that is changed slightly but influences the light and/or signal transmission. Furthermore, white light sensors having different measurement regions and different focus distances exist.